In the example, you may be tempted now to place some blocks in the middle of the first two rows to break up the 10-letter words in rows 1 and 2. Don't, but remember that somewhere between column 4 and 7, you'll want to do that as you proceed with the next step.
Now go to the middle of the grid and fill in blocks to get a nice, aesthetically pleasing shape that breaks up the grid and gives you a nice variability in word length. Most of the time, you'll want to avoid putting a block in the central square. This is also the time to think about placing blocks to make sure you can fill the grid. For example, if you have a J in a theme entry, you might want to place a block such that an entry will begin with the J (roughly half of all words with a J start with the J and there are more than 100 times as many words that start with a J than end with one) Similarly, if you have an X, you're better off if the X is in the middle of an entry. Proceed outward until you get back to the outside and can break up those long areas you started with.
Some other general guidelines:
- Diagonal runs of blocks are generally easier to deal with (both in layout and in fill) than horizontal and vertical runs.
- Most 15x puzzles break up into 9 large sections, which are connected to adjacent sections by 2-3 longer words.
- Avoid small isolated sections of words with only "one way in", even if that way in is a theme entry. It is ok to have both ways in be through the same cell with both a horizontal and vertical way in.
- "Cheater" blocks are blocks which are not required by the theme and, if removed, do not affect the word count. You should avoid them when possible. Don't put them in initially and only put them in if you need them for your fill. I find I can almost always avoid cheaters by rearranging my theme entries or moving other blocks.
- All rules, even symmetry rules, can be violated, but the more significant the rule, the better reason you need. You can't just violate a rule because you couldn't find matching theme entries or you liked the fill words better.
- If you find your puzzle unfillable, don't give up. Oftentimes, moving a few blocks or swapping two theme entries of equal length (if you can do that with the theme you have) can make the difference you need.